Children in the same family share an environment of conditions and relationships; at the same time each child in the family has a unique environment. What and how much is shared and nonshared, what factors in the family and the children contribute to the environments, and how these shared and nonshared environments relate to the development trajectories of siblings are questions addressed in this research. Siblings are compared on variables that index the child's experience as a family member in terms of security, acceptance, and support. The data are observations and interviews. Data are obtained in early, middle, and late childhood.